1. Field of the Invention
The present invention relates to a device for driving a load such as a cooling fan motor mounted on an automotive vehicle and to a method of driving such a load.
2. Description of Related Art
A motor for driving a cooling fan or a blower is mounted on an automotive vehicle. An example of a device for driving such a motor is shown in FIG. 14. Between a plus terminal of a battery 1 and a ground, a fuse 2, a DC motor 3 (a fan-motor), an N-channel power MOSFET 4 (a switching element) and a current detecting resistor 5 are connected in series. The DC motor 3 drives a cooling fan (not shown). The load-driving device 11 includes the power MOSFET 4, the current detecting resister 5, an input signal circuit 6, a driving circuit 7, a load voltage detector 8 (Vm detector), a load current detector 9 (Im detector) and a control circuit 10.
A control signal S1 is fed to the input signal circuit 6 from an electronic control unit (ECU, not shown) that controls operation of an on-board air-conditioner. The air-conditioner ECU outputs the control signal S1 in a form of a pulse-width-modulated signal (PWM) on a carrier wave of 5 kHz, for example. The input signal circuit 6 performs a frequency-to-voltage conversion (F/V conversion) to obtain a voltage signal to be fed to the driving circuit 7. The driving circuit 7 outputs a driving signal according to the voltage signal supplied from the input signal circuit 6. The driving signal is fed to a gate of the FET 4 which controls a load voltage Vm imposed on the motor 3 according to the driving signal fed from the driving circuit 7. The load voltage Vm is expressed in a formula: Vm=(+B)−Vm(−), where (+B) is a terminal voltage of the battery 1, and Vm(−) is a drain voltage of the FET 4 or a voltage at a minus terminal of the motor 3.
The load voltage Vm is controlled to a target load voltage Vt based on the driving signal fed from the driving circuit 7 to the FET 4. A load current detector 9 (Im detector) is connected across the current detecting resistor 5 having a resistance r, and the load current detector 9 detects an amount of current flowing through the resistor 5 based on a voltage across the resistor 5. A voltage Vr (a voltage across the resistor 5 representing the load current Im) is fed to the control circuit 10. The control circuit 10 functions as a protecting circuit for preventing the FET 4 from being damaged due to over-current flowing therethrough. For example, the control circuit 10 gives a command to the driving circuit 7 to decrease the level of the driving signal fed to the FET 4 to thereby limit the load current Im, when the load current Im detected by the current detector 9 becomes larger than a predetermined threshold level. An example of the circuit for protecting the FET 4 from over-current is shown in JP-B2-8-34222.
The device for driving the fan-motor 3 has to supply a certain amount of current to the motor when the motor is temporarily locked due to a low temperature to break the frozen state by giving a torque to a rotor of the motor. Therefore, the amount of the load current is limited only to a certain level for protecting the FET 4 from over-current in the conventional device. However, in the case where the motor is short-circuited and the load current reaches a very high level, there is a possibility that the FET is not sufficiently protected in the conventional device.